A hard disk drive typically comprises a rotating data storage disk 10 and a read/write head transducer assembly including a slider 12 and an electromagnetic read/write transducer unit carried by the slider 12. A plan view of a disk drive of this type is shown in FIG. 1. The slider 12 is positioned radially relative to the rotating disk 10 by a head arm assembly 15, so that write and read elements follow a data track (dt) defined on a storage surface of the disk 10, there being a multiplicity of available concentric data track locations on the surface. The head arm assembly is moved to track (dt) by e.g. a rotary voice coil motor, not shown in FIG. 1.
During rotation of the disk 10, the slider 12 "flies" upon an air film bearing in very close proximity to a data storage surface of the disk. The slider 12 is mounted to a head arm 15 via a gimbal 16 (shown in FIG. 7) and a load beam or spring 17, which applies a predetermined preload force to the slider 12 through the gimbal 16, to urge the slider 12 toward a storage surface of the disk 10. Boundary layer airflow generated by disk rotation relative to the slider creates an air film bearing which enables the slider to "fly" in very close proximity to the disk surface. Because of gimbal mounting, in flight the slider 12 is free to "pitch" in the sense that the leading edge and trailing edge thereof move toward and away from the storage surface in a pitching motion. The slider 12 is also free to "roll" or rotate about the gimbal from side to side. Nominally, the disks 10 and the head-arm E-block 15 are mounted to a base 13 which may be a casting or stamping of e.g. aluminum alloy such that they nominally rotate in parallel planes. As thus far described, the disk drive of FIG. 1 is conventional. However, as will be explained hereinafter, the FIG. 1 disk drive includes a roll-biased head suspension for reduced track misregistration in accordance with principles of the present invention.
Disk drive designs have been characterized by increases in performance and data storage capacity. Performance has been increased by increasing the rotational speed of the disks, from e.g. 3600 RPM to 7200 RPM, or faster. Faster disk rotation provides the benefit of reducing average latency to a data record in a data track. Faster disk rotation has the undesirable drawback of potentially exciting one or more disk resonance modes by virtue of increased air turbulence, or by coupling vibrations through spindle bearings, for example. One consequence of disk resonance is that the disk 10 distorts out-of-plane during disk rotation. Disk vibrations, including but not limited to those occurring at disk resonance frequencies, have contributed to misalignment between the data transducer and a circular data track on the disk surface being followed by the data transducer. This misalignment is known in the art as track misregistration or "TMR".
Data storage capacities have been increased by reducing data track widths, and by increasing the number of tracks per disk. With smaller tracks which are closer together, sensitivity of TMR to disk out-of-plane movements from whatever cause has increased.
An article by J. S. McAllister entitled: "The Effect of Platter Resonances on Track Misregistration in Disk Drives", Journal of Sound and Vibration, January 1996, pp. 24-28, has attributed the disk vibrations to disk resonances driven primarily by internal windage excitation within the head-disk assembly ("HDA") and describes a correlation between the vibrations and TMR. This article further reports that the vibrations manifested by individual disks within the HDA are a characteristic common to all widely used 3.5 inch diameter aluminum alloy disk media. The article also reports that the vibrational behavior is dominated by disk material properties and geometry, and not by the spindle, enclosure or other structural design of the HDA. Moreover, the article does not address the kinematics and structural causes of disk-vibration-induced TMR and it presents no solutions to correct for the resultant TMR.
Observations and measurements made by the present inventors have confirmed the general vibrational behavior of 3.5 inch diameter aluminum storage disks noted by McAllister in the above article. Also, the inventors have noted disk out-of-plane movements and deflections being telegraphed by vibrations occurring in the disk spindle assembly. These effects are difficult to control, and impossible to eliminate altogether. Further, because of the lack of true planarity between disks and head arms discussed above, the present inventors have observed differences in TMR, depending upon whether a particular data transducer is "up-facing" or "down-facing" relative to a disk storage surface. Specifically, in at least one family of disk drives up-facing transducers have been observed to have lower TMR than down-facing transducers in the presence of disk out-of-plane vibrations and motions. Also, the TMR effect has been observed to become worse as one observes from an inside diameter track to an outside diameter track of a storage disk within the HDA.
As a disk moves out of its nominal rotational plane in response to a deflecting force or vibrational excitation, the disk simultaneously deforms or rotates in such a way as to displace the data track recorded on the disk surface in both radial and out-of-plane directions relative to the base 13. Also, simultaneously, the slider and read/write head is moved off-track due to the fact that it is attached to a compliant suspension/head gimbal assembly, and therefore follows the disk contour. As the slider follows the radial and out-of-plane motion of the disk 10, the slider moves off of track.
Thus, we say that the radial and out-of-plane movements of the disk are "coupled" to the TMR parameter. This coupling is present in a disk drive in which the disks 10 and actuator 15 rotate in parallel planes. In disk drives in which the disks 10 and actuator 15 rotate in planes slightly out of parallelism, due to manufacturing tolerances, the disk movements may result in differential TMR between the down-facing transducers and the up-facing transducers.
One approach to solving the presently discussed problem with disk distortion would be to stiffen the storage disk to reduce vertical distortion, by making the disk (nominally 0.8 mm thick) thicker, with a negative impact on the overall achievable disk drive Z height dimension (measured in the direction of disk spindle elevation). Another approach would be to make the disks with either a smaller outside diameter with consequent reduced data storage capacity, or of a material having a greater stiffness, such as glass or ceramic, but at increased cost. A third approach might be to form the storage disk as a composite laminar construction including an inner constrained layer of suitable damping material, only realizable at much greater prime cost per disk.
Thus, a hitherto unsolved need has remained for a method for reducing TMR caused by out-of-plane disk motions and distortion in a manner enabling continued use of conventional disks and heads and with minimum additional expense.